Target and Fat Halo Signs of Bowel

Target Sign

The target sign refers to the enhanced CT appearance of edematous bowel, which demonstrates three layers:

• Inner: Contrast-enhanced inner mucosa
• Middle: Submocosal edema
• Outer: Muscularis propria and/or serosa

The target sign may also be see on nonenhanced CT when the edema is severe.

Differential considerations include:

• Inflammatory bowel disease (Crohn disease and ulcerative colitis)
• Ischemic bowel disease
• Intramural intestinal hemorrhage
• Vasculitis (Henoch-Schönlein purpura)
• Infection (infectious and pseudomembranous colitis)
• Radiation damage (radiation enteritis colitis)
• Portal hypertension

Fat Halo Sign

A similar sign is the fat halo sign, in which there are also three layers:

• Inner: Inner mucosa
• Middle: Fatty infiltration of the submucosa with Housnfield units lower than -10.
• Outer: Muscularis propria and/or serosa

The inner mucosal layer can be appreciated even without intravenous contrast due to the contrast between the soft tissue of the mucosa and the underlying low-attenuation fat.

Differential considerations include:

• Chronic inflammatory bowel disease (Crohn disease and ulcerative colitis)
• Cytoreductive therapy.
• Graft vs host disease.
• Normal appearance of underdistended bowel.

References

• Ahualli J. The target sign: bowel wall. Radiology. 2005 Feb;234(2):549-50.
• Ahualli J. The fat halo sign. Radiology. 2007 Mar;242(3):945-6.